Liquid crystals are useful for electronic displays because light travelling through a thin film of liquid crystal is affected by the birefringence of the film, which can be controlled by the application of a voltage across the film. Liquid crystal displays are desirable because the transmission or reflection of light from an external source, including ambient light, can be controlled with much less power than is required for luminescent materials used in other displays. Liquid crystal displays are now commonly used in such applications as digital watches, calculators, portable computers, and many other types of electronic equipment where the need exists for long-lived operation, with very low voltage and low power consumption. In particular, portable computer displays benefit from liquid crystal displays where display power utilization must be minimized to permit the battery to operate for as long a period of time as possible before recharging, while allowing the majority of the battery utilization to be directed toward computational efforts.
When viewed directly, a liquid crystal display provides a high quality output. However, at large viewing angles, the image degrades and exhibits poor contrast. This occurs because liquid crystal cells operate by virtue of the birefringent effect exhibited by a liquid crystal medium which includes a large number of anisotropic liquid crystal molecules. Such a material will be positively uniaxially birefringent (n.sub.195 &gt;n.sub..parallel. i.e., the extraordinary refractive index is larger than the ordinary refractive index) with the extraordinary refractive index associated with the alignment of the long molecular axes. The phase retardation effect of such a material on light passing through it inherently varies with the inclination angle of the light, leading to a lower quality image at large viewing angles. By introducing an optical compensating element in conjunction with the liquid crystal cell, however, it is possible to correct for the unwanted angular effects and thereby maintain higher contrast at larger viewing angles than otherwise possible.
The type of optical compensation required depends on the type of display which is used. In a normally black display, the twisted nematic cell is placed between polarizers whose transmission axes are parallel to one another and to the orientation of the director of the liquid crystal at the rear of the cell (i.e., the side of the cell away from the viewer). In the unenergized state, no applied voltage, normally incident light from the backlight is polarized by the first polarizer and in passing through the cell, has its polarization direction rotated by the twist angle of the cell. The twist angle is set to 90.degree. so that the light is blocked by the output polarizer. Patterns can be written in the display by selectively applying a voltage to the portions of the display which are to appear illuminated.
However, when viewed at large angles, the dark (unenergized) areas of a normally black display will appear light because of angle dependent retardation effects for light passing through the liquid crystal layer at such angles, i.e., off-nominal incidence light senses an angle-dependent change of polarization. Contrast can be restored by using a compensating element which has an optical symmetry similar to that of the twist cell, but which reverses its effect. One method is to follow the active liquid crystal layer with a twist cell of reverse helicity. Another is to use one or more A-plate retarder compensators. These compensation methods work because the compensation element shares an optical symmetry with the twisted nematic cell; both are uniaxial birefringent materials having an extraordinary axis orthogonal to the normal light propagation direction. These approaches to compensation have been widely utilized because of the ready availability of materials with the required optical symmetry. Reverse twist cells employ liquid crystals and A-plate retarders are readily manufactured by the stretching of polymers such as polyvinyl alcohol.
Despite the effectiveness of these compensation techniques, there are drawbacks to this approach associated with the normally black operational mode. The appearance of a normally black display is very sensitive to cell gap. Consequently, in order to maintain a uniform dark appearance, it is necessary to make the liquid crystal cell very thick, which results in unacceptably long liquid crystal response times. In addition, the reverse twist compensation technique requires the insertion of a second liquid crystal cell into the optical train, adding significant cost, weight, and bulk to the display. For these reasons, it is highly desirable to compensate a normally white display in order to avoid these disadvantages.
In a normally white display configuration, the 90.degree. twisted nematic cell is placed between polarizers which are crossed, such that the transmission axis of each polarizer is parallel to the director orientation of the liquid crystal molecules in the region of the cell adjacent to it. This reverses the sense of light and dark from that of the normally black display. The unenergized (no applied voltage) areas appear light in a normally white display, while those which are energized appear dark. The problem of ostensibly dark areas appearing light when viewed at large angles still occurs, but the reason for it is different and its correction requires a different type of optical compensating element. In the energized areas, the liquid crystal molecules tend to align with the applied electric field. If this alignment were perfect, all the liquid crystal molecules in the cell would have their long axes normal to the substrate glass. This arrangement, known as homeotropic configuration, exhibits the optical symmetry of a positively birefringent C-plate. In the energized state, the normally white display appears isotropic to normally incident light, which is blocked by the crossed polarizers.
The loss of contrast with viewing angle occurs because the homeotropic liquid crystal layer does not appear isotropic to off-normal light. Light directed at off normal angles propagates in two modes due to the birefringence of the layer, with a phase delay between those modes which increases with the incident angle of the light. This phase dependence on incidence angle introduces an ellipticity to the polarization state which is then incompletely extinguished by the second polarizer, giving rise to slight leakage. Because of the C-plate symmetry, the birefringence has no azimuthal dependence. Clearly what is needed is an optical compensating element, also in C-plane symmetry, but with negative (n.sub..parallel. &gt;n.sub..perp.) birefringence. Such a compensator would introduces a phase delay opposite in sign to that caused by the liquid crystal layer, thereby restoring the original polarization state, allowing the light to be blocked by the output polarizer.
This technique has not been used in the past because it has been difficult or impossible to construct a C-plate compensator with the required optical symmetry. There has been no way found to stretch or compress polymers to obtain large area films with negative C-plate optical symmetry and the required uniformity, nor is it possible to form a compensator from a negatively birefringent crystal such as sapphire. In order for a compensator to be effective, the phase retardation of such a plate would have to have the same magnitude as the phase retardation of the liquid crystal and would also have to have the same magnitude as the phase retardation of the liquid crystal and would also have to change with the viewing angle at the same rate as the change of the liquid crystal's phase retardation. These constraints imply that the thickness of the negative plate would thus be on the order of 10 .mu.m, making such an approach very difficult to accomplish because it would require the polishing of an extremely thin plate having the correct (negative) birefringence while ensuring that the surfaces of the plate remained parallel. Since such displays are relatively large in size, the availability of a negatively birefringent crystal of sufficient size would also be a major difficulty. Compensation techniques have been proposed which utilize crossed A-plate compensators. Such an arrangement, however, cannot produce a compensator with an azimuthal (C-plate) symmetry. Because of these difficulties, the tendency in the art has been to rely on normally black displays, even though the normally white type could produce a superior quality display if an appropriate compensator were available.
Previous attempts at compensator fabrication are described, for example in U.S. Pat, No. 5,138,474, to Fuji Photo Film Co., Ltd., and wherein the technique relied upon for imparting negative birefringence required film stretching. Compensators are designed to improve the viewing angle dependence which is a function of retardation (Re), defined as a product of a birefringence (.DELTA.n) of a film and a film thickness (d). The viewing angle is improved by inserting a film having an optic axis substantially in the normal direction, more specifically, a laminated film of a biaxially stretched film having a negative intrinsic birefringence and a uniaxially stretched film having a positive intrinsic birefringence between a liquid crystal cell and a polarizing sheet. Preferred examples of polymers for use in preparing the stretched films having a positive intrinsic birefringence include polycarbonates, polyarylates, polyethylene terephthalate, polyether sulfone, polyphenylene sulfide, polyphenylene oxide, polyallyl sulfone, polyamide-imides, polyimides, polyolefins, polyvinyl chloride, cellulose and polyarylates and polyesters which have a high intrinsic birefringence prepared by for example, solution casting.
Preferred examples of negative intrinsic birefringence stretched films would include styrene polymers, acrylic ester polymers, methacrylic ester polymers, acrylonitrile polymers and methacrylonitrile polymers with polystyrene polymers being most preferred from the viewpoint of large absolute value of intrinsic birefringence, transparency and ease of processing into films by solution casting.
However, to date, there still is lacking a method for producing negative intrinsic birefringence films without the need of having to resort to physical polymer film stretching to introduce the necessary orientation. Physical drawing of polymer films to achieve the necessary orientation and ordered areas which increase the scattering of light, are difficult to achieve, particularly regarding film uniformity. While on a macroscopic level the films superficially appear to be uniform, this is not the case on a microscopic level. To date, there has been no self-orienting, inherently in-plane oriented negative intrinsic birefringence films which are uniform on a microscopic level.